PROJECT SUMMARY Colorectal cancer (CRC) remains a major public health issue, with approximately 135,000 new cases and 50,000 deaths from this disease per year in the US. The incidence of CRC rises sharply after the age of 40. Dysbiosis of the gut microbiome and elevated inflammation are established risk factors for CRC. Similarly the gut microbial composition changes with aging and becomes more pro-inflammatory. It remains unclear, however, whether age-related changes in the gut microbiome are causally involved in the increase in CRC risk seen with age. Our long term goal is to identify microbes that suppress colorectal carcinogenesis and to promote their growth and/or administer them, either singly or in combination, to reduce the risk of CRC in at-risk populations such as elder adults or the obese. The objective of this application is to determine whether age-related changes in the gut microbiome promote CRC formation. In our recent studies we have observed the normal gut commensal bacterium Parabacteroides distasonis (Pd) to suppress pro-inflammatory TLR4 signaling and cytokine release in intestinal epithelial cell lines. Moreover administration of this bacterium completely blocked obesity-driven colon tumor formation in mice. Thus a secondary objective is to determine whether Pd can attenuate tumorigenesis driven by dysbiotic microbial communities derived from elderly mice. We hypothesize that the microbiome of aged animals will promote CRC formation and that the anti-inflammatory bacterium Pd will attenuate this effect. The specific aims of the study are therefore to: i) determine whether microbial communities from aged mice can promote tumorigenesis to a greater extent than communities from young or adult mice when transplanted into young germ-free mice and, ii) determine whether Pd can suppress tumorigenesis in mice inoculated with microbes from aged donor mice. These aims will be addressed by conducting a mouse study with a 3 x 2 factorial design. Briefly, germ-free mice will be inoculated with the stool from young (6 wk), adult (24 wk) and old (72 wk) conventional mice and will then be randomized to receive regular diet or diet plus freeze- dried Pd (0.04% w/w). All recipient mice will then be dosed with the colorectal carcinogen azoxymethane and tumor burden compare between groups. Understanding the contribution of changing gut microbial communities to the increased risk for CRC with age will allow us to tailor interventions to prevent this disease in this at-risk population. Such interventions could involve cocktails of bacteria selected for their ability to suppress inflammation and tumorigenesis. This endeavor is consistent with the mission of the NCI to develop the knowledge base that will lessen the burden of cancer in the United States and around the world.